1. Field of the Invention
The present invention relates to configuration of a rabble useful for moving material, and to systems and methods utilizing such rabbles for moving material for example, during thermolytic processes in a rotary hearth. More specifically, it relates to methods and apparatus utilizing such rabbles for feeding, conveying and discharging coke during a calcining operation.
2. Prior Art
Oven systems utilizing relative motion between a hearth and rabbles have long been used for heating and calcining material. Such systems normally include, for example, a heating chamber including a roof and cylindrical sidewalls, a circular hearth, a rabble system, a mechanism for imparting relative motion between the rabbles and the hearth, suitable means for the admission of to-be-treated material into the oven, and other means for discharging treated material from the oven. Rabble systems have usually included a plurality of rabbles supported above and extending closely adjacent to the floor of the hearth for engaging and advancing the materials thereon from the area of entry to the point of discharge in response to relative motion between the floor of the hearth and the rabbles. The motion may be provided either by rotating the floor of a hearth while maintaining the rabbles stationary, or revolving the rabbles while maintaining the floor stationary. Such systems may also include ports for the admission of gases, and an exhaust system for combustion by-products and spent gases.
In such prior art rotary hearth systems, the material to be treated has normally been admitted either in the area near the center of the hearth floor and transported by the rabbles to the peripheral edge of the hearth for discharge, or admitted in the area near the peripheral edge of the heath floor and transported by the rabbles to the center of the hearth for discharge. Calcining in rotary furnaces is taught, for example, in U.K. Pat. No. 1,055,857 and U.S. Pat. No. 3,448,012.
In most of such prior art systems, the rabbles have generally been arrayed with an active face oriented to contact and progressively move material on the floor from its point of entry to one or more discharge outlets. The angular orientation and general configuration of prior art rabbles have generally been considered non-critical. The only concern as to the orientation and configuration of the rabbles has been that their active faces contact and move material across the floor in the desired direction during relative motion. The rabbles themselves have generally tended to be plate like and flat in structure. In some instances, linear rabbles have been dished. Generally, their leading edges have been rectilinear, although U.S. Pat. No. 3,475,286 discloses a plow shaped rabble. The angle at which the leading edge of a rabble and the active face of a rabble is oriented with respect to the flow of material on the floor is not known to have previously been considered critical. However, the practice has appeared to favor orientation angles of about 20.degree. to 45.degree. to the flow of material at the leading edge and active face. It is also noted that in the prior art, U.S. Pat. Nos. 319,180; 740,103 and 1,878,581 have disclosed rabble systems in which the angle of the rabbles is variable. However, they disclose no preferred angle, and specifically they disclose no angles in the range of about 0.degree. to about .+-.13.degree..
Rotary material handling systems utilizing curved elements have been taught in the prior art. U.S. Pat. No. 319,180 shows a system in which the innermost scraper is curved. However, the active face of this scraper is convex to the direction of material flow. U.S. Pat. Nos. 1,503,234 and 2,973,565 disclose the use of a curved distributing spoke or a scraper, respectively, which traverses the entire radius of the floor to circumferentially distribute or level material on the floor without moving the material in any radial direction. U.S. Pat. No. 3,905,757 uses chains or bars in much the same manner to achieve much the same function.
U.S. Pat. Nos. 3,470,068 and 3,475,286 disclose the use of pairs of staged leading and following rabbles. In these systems, each of the rabbles is at a different height, both rabbles in the pair circumscribe the exact same path on the floor and the rabbles are not in contact with one another to form a convex surface. The leading edges and active faces of the rabbles are not set at low angles in these systems.
Now, in the practice of the prior art, wherein the initial body portions extending from the leading edges of the rabbles are oriented at conventional angles greater than about .+-.20.degree., as the rabbles proceed through the material on the floor there is a tendency for the materials to build-up in front of the leading edge of each rabble. This build-up of material causes substantial variations in the height of material on the floor. In a hearth, the variations in height in turn result in uneven movement of the material across the hearth. During calcination of material, such as coke, it is important that the material be heated completely and uniformly. Where the material is distributed unevenly on the hearth, it does not heat uniformly. Furthermore, where the rabbles are closely adjacent to one another, there is a tendency for the built-up material in front of the rabbles to join from rabble to rabble and form a "bridge" of material. Such bridges tend to block the flow of material to the rabbles and thus make the rabbling action inefficient.
Material build-up in front of the leading edges of rabbles is reduced in accordance with the teaching of U.S. Patent Application Ser. No. 756,161, filed simultaneously with the present application. In that application, it is taught that by orienting the trailing edge of a rabble at an angle in the range of about .+-.2.degree. and about .+-.13.degree., build-up of material in front of the leading edge of the rabbles is reduced. Unfortunately, utilizing rabbles with such trailing edge orientation does not always avoid bridging problems between closely adjacent rabbles.